Demon Doors
One of more intriguing, yet puzzling encounters you'll have in Fable: The Lost Chapters is the Demon Door. Each of these stony constructs is affixed with a demonic face and will bar your way to an area either filled with valuable treasure, or simply required to continue a quest. When you approach the door, the face will speak and provide you with a riddle of sorts that you will have to solve before being able to pass through. No guesswork here, though, since all of the specifics on getting through each one is detailed below.
Demon Door Demon Door #1 (Heroes' Guild)

This should be the first Demon Door you encounter, especially since there is actually a "Demon Door race" that you can participate during your first years at the guild. You won't be able to enter it, however, until you receive the Lamp at your graduation from the guild and use it in front of the stony face. Once inside, you'll be able to scoop up an Elixir of Life, a Howl Tattoo, and a couple of books.
Demon Door Demon Door #2 (Greatwood Gorge)

This door requires you to exhibit a display of "evil", and shouldn't pose much of a problem unless you're planning on staying strictly "good" through the game. To pass through, you must murder several commoners in front of it (traders, henchmen, etc), eat ten Crunchy Chicks, or already have horns and billowing red smoke. The reward for your vile efforts is a Legendary weapon - Wellow's Pickhammer.
Demon Door Demon Door #3 (Greatwood Caves)

To crack this door, you'll need a Combat Multiplier of 14+. To achieve this, build up your multiplier to 20+ in the Hobbe Cave and then fight the Troll that spawns just outside the door to get yourself back to 14+. It's tricky, but using a combination of Physical Shield and Slow Time should get you there. Once complete, head inside to grab another Legendary weapon - The Cutlass Bluetane.
Demon Door Demon Door #4 (Darkwood Marshes)

Once you have the hang of combat, give this door a try. The door will ask if you are willing to battle its "guardians", and if you accept, it will spawn a small army of Hobbes - including the more powerful Lieutenants. Not only will you rack up some nice experience, but you'll also score a set of Will Dark Armor (upper robe, lower robe, gloves, boots) inside the hidden chamber.
Demon Door Demon Door #5 (Barrow Fields)

Before reaching this door, be sure to stock up on plenty of Apple Pie or Red Meat. Why, you ask? Well, this particular Demon Door will not allow you to enter its domain unless you're considered "obese". To pack on the pounds, you'll need to devour close to 30 pies or pieces of meat at full health. A Will Master's Elixir (a potion that grants a permanent mana increase) awaits you inside.
Demon Door Demon Door #6 (Witchwood Stones)

This particular Demon Door is directly related to the Archaeologist Quest given to you by Maze in the Heroes' Guild. To enter, spell the door's name by swinging your weapon at the four pillars not far down the path in this order: H-I-T-S. Once complete, head inside to speak with the Archaeologist and to loot a Health Augmentation, Chainmail Leggings, Resurrection Phial, Howl Tattoo, and more.
Demon Door Demon Door #7 (Knothole Glade)

Just outside the main gate into Knothole Glade resides yet another Demon Door. To reap the reward inside this door, you must shoot it with your bow - hard. Unless you have trained your Accuracy quite a bit or have access to Multi Arrow, you probably won't be able to strike it hard enough. However, once you do, an Elixir of Life can be found inside.
Demon Door Demon Door #8 (Grey House)

Once you've hacked your way through all of the undead in the area and reached this door, it will ask that you are married to a "posh wife with a title" before allowing you to enter. There's only one woman that fits that description, and that would be Lady Grey of Bowerstone. Once you're hitched to her, you can score a Legendary weapon inside called Ronok the Axe.
Demon Door Demon Door #9 (Rose Cottage)

Perhaps a tad bit strange, this door is looking for a sign of affection from you. No, you don't have to humiliate yourself... just give the door a gift like a Red Rose or some Chocolates. Doing so will bring a smile to its "face" and get you access to the hidden area behind it, where you'll find a full set of Will Bright Armor (upper robe, lower robe, gloves, boots) up for grabs.
Demon Door Demon Door #10 (Abandoned Road)

This will most likely be one of the last doors you'll get access to, because it requires you to don three different suits of armor - one of which is very tough to obtain. First, you must dress up as a Knight in a full suit of Bright Plate Mail, then a Wizard in a full suit of Will Dark Armor, and finally a Rogue in a full set of Bandit Armor to obtain the Legendary Dollmaster's Mace inside.
Demon Door Demon Door #11 (Lychfield Graveyard)

This door is not optional, it is required to continue your quest to rescue your mother. To get past it, scour the entire graveyard (including ponds and graves) for the four pieces of Nostro's armor (helmet, sword, shield, and armor) and place them inside the sarcophagus in the western crypt. You'll then be able to pass through the door to the Old Graveyard Path.
Demon Door Demon Door #12 (Headsman's Hill)

You will only encounter this Demon Door if you are trying to win Lady Grey's hand in marriage. She will ask you to prove your battle prowess against former Arena champion Thunder. During the battle in Headsman's Hill, Thunder will throw you both off a cliff to a landing below. To get back to Gibbet Woods, the door will require that Thunder has indeed been defeated.
Demon Door Demon Door #13 (The Darkwood Bordello) (TLC)

You'll find this demon door behind the Darkwood Bordello. It'll ask you to demonstrate that you're a "real stud." To satisfy the door, you'll also need to satisfy yourself -- ten times. Conveniently, the bordello is nearby to help you out. Beyond the door you'll find the pimp hat.
Demon Door Demon Door #14 (Lookout Point) (TLC)

This is the Primal Demon Door. It won't "wake up" until after you've defeated Jack of Blades, but after that it will recognize you for the true hero that you are, and it will let you through. Beyond the door you'll have to solve a puzzle so you can claim the Fire Heart.
Demon Door Demon Door #15 (Necropolis) (TLC)

This Demon Door wishes that it was a silver chest. If you give it all of your silver keys, then you'll please it, and it will open for you. Beyond the door you'll find a chest with the legendary weapon The Bereaver inside it.